This proposal is a two-part study aimed at developing radiotracers that map the specific neurons in mammalian brain. The primary goal is to develop an {I-123} labeled tracer that will allow external detection of regional cholinergic neuron density of the human brain by single photon emission tomography. Such as agent would have potential widespread use in Nuclear Medicine for assessing the extent of cholinergic neuron involvement in Alzheimer's disease, olivopontocerebellar atrophy, progressive supranuclear palsy, and in demented patients with Parkinson's disease. We have adopted vesamicol, a potent inhibitor of vesicular acetylcholamine storage, as the design basis for a presynaptic cholinergic nerve marker. Work in our laboratory with a derivative of vesamicol, 5-[I-125] iodobenzovesamicol, has shown it to be a highly specific in vivo marker for the cholinergic neuron of murine brain. A systematic structure-activity study of [I-125] labeled vesamicol analogs will be pursued to determine optimum brain uptake, neuronal specificity, and tracer kinetics. Regions of bulk tolerance will be determined so incorporation of radioiodine will cause minimal perturbation of binding to the vesamicol receptor. A spectrum of hydrophilic groups will be evaluated to modulate lipophilicity within the optimum log P window for brain uptake. Evaluation of promising tracers will include gross regional brain distribution, quantitative autoradiography, pharmacological blocking/receptor saturation studies, and tracer kinetic analyses in rat and monkey, the latter with [I-123] labeled tracer using a newly developed small animal imaging device. The second goal is to develop tracers that map neuron-specific reuptake systems in brain. Initial efforts will focus on dopaminergic neurons using [I-125] analogs of the GBR series of inhibitors. Synthetic efforts will also be directed at radioiodinated inhibitors of the GABA uptake system. The major challenge to successful design of radiolabeled presynaptic uptake inhibitors is to maximize brain uptake without sacrificing neuronal selectivity. Major structural changes of the inhibitor will be made to hopefully counter the lipophilic-enhancing properties of iodine. Bioevaluation of these radiotracers will be essentially as described above for the cholinergic markers.